Generic Name: levothyroxine (Injection route)
Medically reviewed on June 7, 2018
Commonly used brand name(s)
In the U.S.
Available Dosage Forms:
- Powder for Solution
Therapeutic Class: Thyroid Supplement
Uses For Synthroid
Levothyroxine is used to treat hypothyroidism, a condition where the thyroid gland does not produce enough thyroid hormone. Levothyroxine injection can be used as a substitute for the oral dose when a rapid effect is needed and when the oral route is not allowed .
This medicine is available only with your doctor's prescription .
Before Using Synthroid
In deciding to use a medicine, the risks of taking the medicine must be weighed against the good it will do. This is a decision you and your doctor will make. For this medicine, the following should be considered:
Tell your doctor if you have ever had any unusual or allergic reaction to this medicine or any other medicines. Also tell your health care professional if you have any other types of allergies, such as to foods, dyes, preservatives, or animals. For non-prescription products, read the label or package ingredients carefully.
Appropriate studies performed to date have not demonstrated pediatrics-specific problems that would limit the usefulness of levothyroxine in children .
Appropriate studies performed to date have not demonstrated geriatrics-specific problems that would limit the usefulness of levothyroxine in the elderly. However, elderly patients are more likely to have age-related heart and blood vessel problems, which may require caution in patients receiving levothyroxine .
Studies in women suggest that this medication poses minimal risk to the infant when used during breastfeeding.
Interactions with Medicines
Although certain medicines should not be used together at all, in other cases two different medicines may be used together even if an interaction might occur. In these cases, your doctor may want to change the dose, or other precautions may be necessary. When you are receiving this medicine, it is especially important that your healthcare professional know if you are taking any of the medicines listed below. The following interactions have been selected on the basis of their potential significance and are not necessarily all-inclusive.
Using this medicine with any of the following medicines is usually not recommended, but may be required in some cases. If both medicines are prescribed together, your doctor may change the dose or how often you use one or both of the medicines.
Using this medicine with any of the following medicines may cause an increased risk of certain side effects, but using both drugs may be the best treatment for you. If both medicines are prescribed together, your doctor may change the dose or how often you use one or both of the medicines.
- Conjugated Estrogens
- Esterified Estrogens
- Lanthanum Carbonate
Interactions with Food/Tobacco/Alcohol
Certain medicines should not be used at or around the time of eating food or eating certain types of food since interactions may occur. Using alcohol or tobacco with certain medicines may also cause interactions to occur. The following interactions have been selected on the basis of their potential significance and are not necessarily all-inclusive.
Using this medicine with any of the following may cause an increased risk of certain side effects but may be unavoidable in some cases. If used together, your doctor may change the dose or how often you use this medicine, or give you special instructions about the use of food, alcohol, or tobacco.
Other Medical Problems
The presence of other medical problems may affect the use of this medicine. Make sure you tell your doctor if you have any other medical problems, especially:
- Adrenal insufficiency or Addison's disease (untreated) or
- Heart attack or
- Thyrotoxicosis (overactive thyroid)—This medicine should NOT be used in patients with any of these conditions .
- Clotting disorder or
- Diabetes or
- Heart disease (history of) or
- Other adrenal gland problems or
- Underactive pituitary gland—Use with caution. Dosage adjustment may be needed .
Proper Use of Synthroid
The dose of this medicine will be different for different patients. Follow your doctor's orders or the directions on the label. The following information includes only the average doses of this medicine. If your dose is different, do not change it unless your doctor tells you to do so.
The amount of medicine that you take depends on the strength of the medicine. Also, the number of doses you take each day, the time allowed between doses, and the length of time you take the medicine depend on the medical problem for which you are using the medicine.
- For injection form:
- For the treatment of hypothyroidism:
- Adults and teenagers—50 to 100 micrograms (mcg) injected into a muscle or into a vein once a day. People with very serious conditions caused by too little thyroid hormone may need higher doses.
- Children—The dose is based on body weight and must be determined by your doctor .
- For the treatment of hypothyroidism:
Precautions While Using Synthroid
It is very important that your doctor check your progress at regular visits. This will allow your doctor to see if the medicine is working properly and to decide if you should continue to take it. Blood and urine tests will be needed to check for unwanted effects .
Levothyroxine should not be used for the treatment of obesity or for the purpose of losing weight. This medicine is ineffective for weight reduction and when taken in larger amount, it may cause more serious medical conditions .
Synthroid Side Effects
Along with its needed effects, a medicine may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention.
Get emergency help immediately if any of the following symptoms of overdose occur:
- Changes in appetite
- changes in menstrual periods
- chest pain
- fast or irregular heartbeat
- hand tremors
- leg cramps
- sensitivity to heat
- shortness of breath
- trouble sleeping
- weight loss
Other side effects not listed may also occur in some patients. If you notice any other effects, check with your healthcare professional.
Call your doctor for medical advice about side effects. You may report side effects to the FDA at 1-800-FDA-1088.
See also: Side effects (in more detail)
Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.
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